Such a process and device is known from DE 33 45 937 A1 where the connecting line between an evaporation device and a condensation device cannot be shut off. To produce the desirable partial vacuum, the evaporation device is connected with at least one tank which is filled with untreated liquid, and provided at a height that is at least by the water column producible by the ambient air pressure above a water level, which tank is provided with a downpipe that is immersed in the water level and can be shut off. By opening the downpipe, liquid flows off causing a partial vacuum in the evaporation device if venting thereof is prevented, which partial vacuum is in this case passed on to the condensation device via the open connecting line. The required location of the tank at a high level and the immersing of the bottom end of the downpipe in an existing water level result in a relatively large overall height of the device and high constructional expenditure. Nevertheless, due to the existing connection between the evaporation device and the condensation device, the obtainable partial vacuum is relatively small. In addition, the liquid in which the downpipe is immersed may contain gas bubbles produced by gas emission etc., which may accumulate at the bottom end of the downpipe and then rise in it, which may result in a further deterioration of the obtainable partial vacuum.